Method and apparatus for purifying exhaust air of a dryer apparatus

ABSTRACT

A method and apparatus for dryers for products containing volatile hydrocarbons such as impregnated hard board, electrical insulation having insulating varnish, aluminum sheets coated with varnish, etc.; the products being dried normally producing, during drying, gaseous hydrocarbons which, for ecological reasons, cannot be discharged directly into the atmosphere but which must be &#34;treated;&#34; the method and apparatus concerning a two-sectioned dryer in series in which the material being dried, passes from section to section; in the first section the more volatile hydrocarbons are driven off and are &#34;after-burned&#34; with the heat of combustion being recovered and used to heat the dryer sections; the second section having less volatile hydrocarbons driven off which are &#34;washed&#34; or condensed; the hydrocarbons precipitating out during &#34;washing&#34; being used as a fuel to assist in the &#34;after-burning&#34; of the more volatile hydrocarbons; the washed exhaust from the second section and exhaust gases from &#34;after-burning&#34; of the more volatile hydrocarbons being combined after washing and after-burning for dehumidification of the washed exhaust and discharged directly into the atmosphere.

United States Patent Vits METHOD AND APPARATUS FOR PURIFYING EXHAUST AIROF A DRYER APPARATUS Apr. 8, 1975 Prinwry E.\'aminerl(enneth W. SpragueAssistant Examiner-Larry l. Schwartz Attorney, Agent, or FirmHolman &Stern [57] ABSTRACT A method and apparatus for dryers for productscontaining volatile hydrocarbons such as impregnated hard board,electrical insulation having insulating varnish, aluminum sheets coatedwith varnish, etc.; the products being dried normally producing, duringdrying, gaseous hydrocarbons which, for ecological reasons, cannot bedischarged directly into the atmosphere but which must be treated;" themethod and apparatus concerning a two-sectioned dryer in series in whichthe material being dried, passes from section to section; in the firstsection the more volatile hydrocarbons are driven off and areafter-burned" with the heat of combustion being recovered and used toheat the dryer sections; the second section having less volatilehydrocarbons driven off which are washed" or condensed; the hydrocarbonsprecipitating out during washing" being used as a fuel to assist in theafterburning" of the more volatile hydrocarbons; the washed exhaust fromthe second section and exhaust gases from after-burning of the morevolatile hydrocarbons being combined after washing and afterburning fordehumidification of the washed exhaust and discharged directly into theatmosphere.

[75] Inventor: I-lilmar Vits, Lcichlingen, Germany [73] Assignee:Vits-Maschinenbau GmbH,

Langenfeld, Germany [22] Filed: Jan. 4, 1974 [21] Appl. No.: 430,94l

[30] Foreign Application Priority Data Jan. 5, 1973 Germany 2300381 152]U.S. Cl. 34/32; 34/72; 34/79; 7 23/277 C; 431/5 [51] Int. Cl. F26b 3/00[58} Field of Search 431/5; 34/32, 72, 79, 210; 23/277 C; 110/8 A [56]References Cited UNITED STATES PATENTS 3.314.159 4/1967 Betz 34/723.472,498 10/1969 Price et a1 431/5 3.601.900 8/1971 Erisman et a1...34/79 3,702,039 1 1/1972 Stookey et al 48/111 3,736,111 5/1973 Gardneret al H 48/111 METHOD AND APPARATUS FOR PURIFYING EXHAUST AIR OF A DRYERAPPARATUS FIELD OF THE INVENTION The instant invention concerns methodand apparatus for purifying the exhaust gases and by-products from adryer apparatus for products moving therethrough, wherein said productscontain light and heavy hydrocarbons which become volatile during thedrying phase and are incinerated before their exit into the at mosphere.

BACKGROUND OF THE INVENTION In dryers for such type of products, thedrying air must be retained at a predetermined temperature, for exampleat 160C, so that the light and heavy hydrocarbons become volatile and,mixed with the air, can be exhausted as spent or used air. The heatedgases which are drawn from the dryer are replaced by a correspondingamount of fresh air, which must be heated to the predeterminedtemperature; this requires a continuous supplying of heat. The fresh airis drawn into a suspension dryer through the dryer inlet and outlet forthe products moving therethrough and through non-sealed points of thesuspension dryer by an extractor-fan or vacuum blower.

In the drying of the aforementioned products of the character involved,there exist certain regulations for the environmental protection underwhich the volatilized hydrocarbons must be rendered almost completelysafe by utilizing an after-burning of the dryer exhaust air.

A first solution for partial recovery of the drying heat, and to renderthe volatilized hydrocarbons safe by means of after-burning, consists ofthe system shown diagramatically in FIG. 1, the exhaust gases ofthedryer are entirely subjected to after-burning and a part of the flue-gaswhich has been heated for example up to 800C, is directed back into thedryer for the purpose of heating the dryer-atmosphere, while theremaining part of the flue-gas is emitted into the open atmosphere.

This method is impractical for a multitude of reasons: firstly, a largepart of heat is lost with the flue-gas which is being discharged intothe free atmosphere; secondly, the installation required for such aprocess is costly since on one hand the large volumes. large-column,long flue-gas pipes and the large valves must be heatresistant onaccount of the high temperatures of the flue-gas and must therefore bemanufactured from expensive material, such as, for example,chrome-nickel steel, and on the other hand, it is difficult to retainthe under-pressure in the dryer which is required for a satisfactorysuspension of the products due to the flue-gas supplied in addition tothe fresh-air and the additional sealing points at the connections ofthe flue-gas conduits which are distributed throughout the dryer.

Another system to render the polluted drying air safe for theenvironment, and to partially recover the heat with a less expensiveinstallation, consists in the system shown in FIG. 2; wherein theflue-gas of the entire dryer is subjected to an after-burning phase andthe thus produced flue-gas is directed into the open atmosphere throughan economizer, whereby the flue-gas heats a heat-carrier in aneconomizer, for example, a heating-oil, which is supplied to the dryer,in order to retain its oven-temperature at the desired temperaturelevel. This method, due to the low temperature of, for example 300C,does not require conduits of a high heat-resistant steel and ordinarysteel products suffice. On the basis of this special heat-recovery, thedifficulties in retaining the required underpressure in the dryer do notexist since, in addition to the fresh air, no flue-gas is directed intothe dryer; however, in this type of heat-recovery, the heat-loss isstill too high. Additionally, in such type of heat-recovery there existsthe danger that the heating-oil in the economizer may become overheateddue to the flue-gas being heated to approximately 800C.

A further system to render the fluegases safe for the environment and tomore efficiently recover more heat than in the two above-describedsystems of FIGS. 1 and 2, whereby the costs of the installation can beminimized, and whereby difficulties with the retention of the reducedpressure in the dryer can be avoided, consists of the system of FIG. 3in that all of the flue-gas from the dryer is directed to the apparatusfor an after burning process by means of a recuperator, whereby theflue-gases first are directed through the recuperator and then throughan economizer for a heat-carrier (thermo-oil) then the flue-gases areexhuasted into the free atmosphere. As noted in the above example, thereexists also in this method the danger that the heatingoil, which isbeing utilized as the heat-carrier, and which is supplied to the dryerfor heating its atmosphere, may be overheated. In contrast to theearlier mentioned example, this method has, however, the advantage thatthe heat is more efficiently recouped, however, the higher costs forsuch an installation because of the recuperator must be taken intoconsideration.

SUMMARY OF THE INVENTION It is the scope of the instant invention toprovide a method and apparatus of the above-mentioned type in which,under low expenditures, for the installation, a reduced amount of heatcompared to the amount of the above-described methods, is required inorder to purify the exhuasted air.

This problem is solved by the instant invention in that only theflue-gas from a first section of the dryer in the path of the productsmoving therethrough are immediately after-burned, while the flue-gasfrom the second section of the dryer is washed and the residue orprecipitate from this washing phase together with the burnable materialsis burned off with the flue-gas of the first section of the dryer.

In this method, surprisingly, it is possible to remove and burnapproximately percent of light hydrocarbons from the first section ofthe dryer, while approximately 95 percent of the heavy hydrocarbons fromthe second section of the dryer are removed and burned. The inventivemethod distinguishes over the prior art with regard to heat-balance(thermal balance) as well as with regard to the expenditure for theinstallation. Since the flue-gas is after-burned immediately from thefirst section of the dryer, it is possible to maintain the installationon a smaller scale for the after-burning process as well as for theheat-exchanger installed for reheating the dryer air, in contrast toexpenditures of prior art methods in which all of the flue-gases of thedryer are after-burned. Even though the inventive method requires apurifyer or wash column for the fluegas from the second section of thedryer, the costs are by far not as high as the additional costs for adevice for the after-burning and the heat-exchanger, which must processall of the flue-gases emitted from the dryer apparatus. The total heatorthermal-balance in the inventive method compared with the prior artmethods is improved, first since the flue-gas from the first sectioncontains more volatile (readily evaporated) concentrations up to thepermissible limits so that in the afterburning phase only a small amountof supplemental fuel-gas is required; in general, the light and heavyhydrocarbons deliver the heat for the reheating of the dryer, andsecondly since only a part of the column of the exhaust gases receivedfrom the dryer will have to pass as flue-gas, a drop in temperature (orheat gradient) twice the size in comparison to the prior art methods,since it has to deliver the heat which is required for the first and thesecond sections of the dryer.

Since the washed spent air has a high degree of humidity, it isadvantageous to mix the after-burned exhaust from the economizer withthe spent air which has been washed in the wash column before exhaustinginto the open atmosphere so that the entire flue-gas/usedair mixture isdehumidified.

Preferably. heating-oil is utilized as the heat-carrier.

An installation for performing the preferred method comprises a dryerwhich is divided into two adjacentlyarranged chambers disposed in thedirection the products to be dried therein are moved, whereby the firstchamber of the dryer is connected. by means of an exhaust conduit. witha device for after-burning phase, the conduit being series-connected,for the purpose of removing the after-burned spent air, with aheatexchanger for the purpose of reheating the product dryer, and thesecond chamber having a further exhaust conduit which is connected witha wash-column in which the washedout heavy or less volatile hydrocarbonsfrom the sump of which, via a conduit, the washed out heavy hydrocarbonsare supplied to a burner device used for the purpose of after-burningthe more volatile exhaust from the first section of the dryer.

Embodiments of such installations consist in that the heat-exchanger isan economizer using heating-oil as a heat-carrier, whereby the devicefor the after-burning process is provided with means for atomizing theheavier hydrocarbons which are extracted from the sump of thewash-column, wherein the device for the after-burning is provided withauxiliary burners, and wherein the outlet of the heat-exchanger for theafterburned spent air and the outlet gas of the wash column for thewashed spent exhaust gases terminate into a common flue-gas conduit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of oneproposed dryer installation in which flue-gas is directed back to thedryer section;

FIG. 2 is a schematic view of another dryer installation in which aneconomizer is provided between the after-burner and flue-gas outlet andin which a heating oil is used to recover the heat from after-burning ofthe dryer byproducts or exhaust for heating the dryer;

FIG. 3 is a schematic view showing still another proposed installationof an after-burning system utilizing a recuperator intermediate andafter-burner and economizer for treating the spent air of the dryer; and

FIG. 4 is a schematic view of the preferred embodiment of theafter-burning system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The instant invention isexplained hereinbelow by means of a schematic illustration shown in FIG.4. A suspension dryer I is divided into a first section and a secondsection disposed in series in the direction of the path 2 for theproducts to be dried therein, for example, impregnated hard carboard,electrical insulating material comprising base-material and insulatingvarnish, varnished aluminum foil (sheets) etc. The dividing of the dryer1 into a first and a second section is accomplished in such a way sothat in the area of the first section, at temperatures of about C,approximately 95 percent of the light or more volatile hydrocarbons aredriven off, while in the second section at temperatures of about Capproximately 95 percent of the heavy or less volatile hydrocarbonsbecome volatile and are driven off. A mixing of the dryer air from bothsections of the dryer (which occurs with regard to the concentration ofthe hydrocarbons in both parts of the air of the dryer) generally doesnot occur since a trnsformation of the dryer air occurs zonally.

On one hand, the dryer must be provided with openings for the path-entryand path-exit for the purpose of the suspended guiding of the products,and on the other hand there should not exit from the opening thedryerair which is mixed with volatiles. the dryer is maintained underlow pressure. Fresh air is continuoursly supplied to the dryer by meansof the suspended jets or nozzles (not shown) which are necessary for thesuspended transport of the products. In order to enable the retention ofthe under-pressure, spent air is continuously removed or drawn from thefirst and the second section and is directed to an after-burning device3. In addition to that, the heavy hydrocarbons are sprayed as a fuelinto the after-burner device 3 by means of an atomizing nozzle 4. Inorder to permit after-burning, auxiliary burners 5 are provided throughwhich supplemental fuel which is supplied to the device 3 forafterburning exhaust dryer gases. The portion of the extraneous fuelsmust be sufficient to ignite and retain the burning of the light andheavy hydrocarbons in the device 3. At an exotherm of about 400C of thespent air from the first section, it is necessary to supply enoughsupplemental fuel in order to heat the exhaust air to an.

average of about 400C. The light hydrocarbons and the heavy hydrocarbonswhich are contained in the ex-.

haust air of the dryer are then heating the exhaust since they burn atabout 800C. The flue-gases which are heated to this temperature levelare taken from the device 3 and are directed through an economizer 6 towhich they transfer a large part of their heat to a heatcarrier, forexample heating oil, which is circulated via a conduit system 7 to thefirst and the second section of the dryer 1 to thus maintain thedryer-section atmosphere at predetermined drying temperature levels.

The exhaust gases sucked from the second section of the dryer 1,containing the heavy or less volatile hydrocarbons, are moved through awash column 8 in which the heavy hydrocarbons are extracted orprecipitated by means of water which is sprayed thereinto. From a sump 9of the wash column 8, the Iiquified heavy hydrocarbons, as explained,are drained off in the form of a water slurry and supplied to theafter-burner device 3 through nozzle 4 where they are burned. Thecleaned humid exhaust gas of the wash column 8 passes into a mutualflue-gas canal l0, joining the dry flue-gases which exit from theeconomizer 6, whereafter they exit as dehumidified dry flue-gas mixtureinto the open atmosphere.

SUMMARY The dryer utilizes the principal in Fig. 4 of the fact thatcertain hydrocarbons become volatile at about 130C and are drawn off inthe first drying section; accordingly, less heat is required in thissection to remove the more volatile byproducts of the drying process; inthe second section, the product is subjected to about 160C at which theheavier hydrocarbons become volatile. The exhaust from the secondsection is washed where the heavier carbons precipitate to a sump andare pumped to a fuel nozzle to be used as a fuel supplementing the fuelat burner 5 in the after-burner 3. The heat produced at the after-burner3 is recovered at the economizer 6 which includes a circulatingheatabsorbing medium which is redirected back to the dryer for heatingthe respective first and second sections of the dryer; the washedexhaust from washer 8 is combined with the exhaust from the economizer 6for dehumidification and passes from outlet 10 to the atmosphere andcomprises afterburned exhaust (of the lighter hydrocarbons) and washedexhaust (of the heavier hydrocarbons) from respective dryer sections 1and 2.

What is claimed is:

l. A method for purifying the exhaust products from drying apparatus forproducts containing light (lowboiling) and heavy (high-boiling)hydrocarbons which become volative during drying of the products andwhich are burned in a flame prior to being exhausted into theatmosphere, comprising:

moving the product to be dried through two consecutive sections of adryer into contact with a heating medium at a temperature so that light(low-boiling) hydrocarbons are first volatilized mainly in the firstsection and in which heavy (high-boiling) hydrocarbons are volatilizedmainly in the second sectron;

directly after-burning the volative exhaust products of the firstsection;

and washing the volatile exhaust products of the second section.producing a residue including heavy hydrocarbons and directing theresidue together with burnable material of burned exhaust products ofthe first section and burning it with the residue.

2. The method according to claim 1 including the step of supplyingsufficient supplemental fuel to the after-burning for igniting andmaintaining continuous after-burning of both the light hydrocarbons andthe residue including heavy hydrocarbons.

3. The method according to claim 1 including mixing the exhaust productsfrom the second section, purified by washing, with afterburned exhaustproducts and emitting them as a mixture into the atmosphere.

4. The method according to claim 2 including heating in a heat-exchangera heat-absorbing medium by the after-burner and directing the heatedheat-absorbing medium to the respective dryer sections for treating theproduct being dried as it successively passes through the first andsecond sections of the dryer.

5. Apparatus for purifying the exhaust air from a dryer containingvolatile hydrocarbons comprising:

a dryer divided in consecutive chambers through which a product beingdried is moved;

means in said chambers for passing heated air over said product;

individual exhaust conduit means connected to the respective chambersfor drawing off exhaust air containing light (low-boiling) hydrocarbonsand heavy (high-boiling) hydrocarbons as they become volatile in therespective chambers;

after-burning means connected to the conduit receiving the lighthydrocarbons for burning the same; and

washing means connected to the conduit receiving the volatile heavyhydrocarbons for precipitating a residue including heavy hydrocarbons.

6. The apparatus as claimed in claim 5 including heat-exchanger meansconnected in series with the after-burner means for recovering heatproduced at the after-burner means; and

means for directing the recovered heat to the respective dryer chambers.

7. The apparatus as claimed in claim 5 including sump means operativelyconnected to said wash means for recovering the precipitated residueincluding heavy hydrocarbons; and means connected between said sumpmeans and said after-burner means for burning the precipitated residueincluding heavy hydrocarbons and said light hydrocarbons together in theafterburner means.

8. The apparatus as claimed in claim 6 in which said heat-exchangermeans includes a heat-carrier comprising a heat-absorbing fluid, andmeans for circulating the heat-absorbing fluid through theheat-exchanger means and about said dryer chambers.

9. The apparatus as claimed in claim 7 including nozzle means connectedto the after-burner means and said sump means for directing theprecipitated residue including hydrocarbons into the after-burner means.

10. The apparatus as claimed in claim 6 including auxiliary burners insaid after-burner means for providing supplemental igniting fuel tomaintain continuous operation of said after-burner means.

11. The apparatus as claimed in claim 6 including exhaust gas conduitseach respectively connected to one of the washing means and saidheat-exchanger means. and a common flue-gas conduit joining therespective exhaust conduits whereby the wet exhaust gas from the washingmeans and causes products passing through said heat exchanger means aremixed prior to discharge into the atmosphere.

1. A METHOD FOR PURFYING THE EXHAUST PRODUCTS FROM DRYING APPARATUS FORPRODUCTS CONTAINING LIGHT (LOW-BOILING) AND HEAVY (HIGH-BOILING)HYDRCOARBONS WHICH BECOME VOLATIVE DURING DRYING OF THE PRODUCTS ANDWHICH ARE BURNED IN A FLAME PRIOR TO BEING EXHAUSTED INTO THEATMOSPHERE, COMPRISING: MOVING THE PRODUCT TO BE DRIED THROUGH TWOCONSECUTIVE SECTIONS OF A DRYER INTO CONTACT WITH A HEATING MEDIUM AT ATERMPERATURE SO THAT LIGHT (LOW-BOILING) HYDROCARBONS ARE FIRSTVOLATILIZED MAINLY IN THE FIRST SECTION AND IN WHICH HEAVY(HIGH-BOILING) HYDROCARBONS ARE VOLATILIZED MAINLY IN THE SECONDSECTION, DIRECTLY AFTER-BURNING THE VOLATIVE EXHAUST PRODUCTS OF THEFIRST SECTION, AND WASHING THE VOLATILE EXHAUST PRODUCTS OF THE SECONDSECTION, PRODUCING A RESIDUE INCLUDING HEAVY HYDROCARBONS AND DIRECTINGTHE RESIDUE TOGETHER WITH BURNABLE MATERIAL OF BURNED EXHAUST PRODUCTSOF THE FIRST SECTION AND BURNING IT WITH THE RESIDUE.
 2. The methodaccording to claim 1 including the step of supplying sufficientsupplemental fuel to the after-burning for igniting and maintainingcontinuous after-burning of both the light hydrocarbons and the residueincluding heavy hydrocarbons.
 3. The method according to claim 1including mixing the exhaust products from the second section, purifiedby washing, with afterburned exhaust products and emitting them as amixture into the atmosphere.
 4. The method according to claim 2including heating in a heat-exchanger a heat-absorbing medium by theafter-burner and directing the heated heat-absorbing medium to therespective dryer sections for treating the product being dried as itsuccessively passes through the first and second sections of the dryer.5. Apparatus for purifying the exhaust air from a dryer containingvolatile hydrocarbons comprising: a dryer divided in consecutivechambers through which a product being dried is moved; means in saidchambers for passing heated air over said product; individual exhaustconduit means connected to the respective chambers for drawing offexhaust air containing light (low-boiling) hydrocarbons and heavy(high-boiling) hydrocarbons as they become volatile in the respectivechambers; after-burning means connected to the conduit receiving thelight hydrocarbons for burning the same; and washing means connected tothe conduit receiving the volatile heavy hydrocarbons for precipitatinga residue including heavy hydrocarbons.
 6. The apparatus as claimed inclaim 5 including heat-exchanger means connected in series with theafter-burner means for recovering heat produced at the after-burnermeans; and means for directing the recovered heat to the respectivedryer chambers.
 7. The apparatus as claimed in claim 5 including sumpmeans operatively connected to said wash means for recovering theprecipitated residue including heavy hydrocarbons; and means connectedbetween said sump means and said after-burner means for burning theprecipitated residue including heavy hydrocarbons and said lighthydrocarbons together in the after-burner means.
 8. The apparatus asclaimed in claim 6 in which said heat-exchanger means includes aheat-carrier comprising a heat-absorbing fluid, and means forcirculating the heat-absorbing fluid through the heat-exchanger meansand about said dryer chambers.
 9. The apparatus as claimed in claim 7including nozzle means connected to the after-burner means and said sumpmeans for directing the precipitated residue including hydrocarbons intothe after-burner means.
 10. The apparatus as claimed in claim 6including auxiliary burners in said after-burner means for providingsupplemental igniting fuel to maintain continuous operation of saidafter-burner means.
 11. The apparatus as claimed in claim 6 includingexhaust gas conduits each respectively connected to one of the washingmeans and said heat-exchanger means, and a common flue-gas conduitjoining the respective exhaust conduits whereby the wet exhaust gas fromthe washing means and causes products passing through said heatexchanger means are mixed prior to discharge into the atmosphere.